Concept Map

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Paper : 03 Structure and Function of Biomolecules II Module : 18 Fatty Acids

Principal Investigator Dr. Sunil Kumar Khare,Professor Dept. of Chemistry,

I.I.T. Delhi

Content Reviewer:

Paper Co-ordinator and Content Writer

Dr. M.N.Gupta, Emeritus Professor Dept. of Biochemical Engg. and Biotechnology, I.I.T. Delhi

Dr. Sunil Kumar Khare,Professor Dept. of Chemistry,

I.I.T. Delhi

Dr. Prashant Mishra, Professor Dept. of Biochemical Engg. and Biotechnology, I.I.T. Delhi

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Description of Module

Subject Name Biochemistry

Paper Name 03 Structure and Function of Biomolecules II Module Name/Title 18 Fatty Acids

Dr. Vijaya Khader Dr. MC Varadaraj

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Objectives

 To learn about various fatty acids

 To understand how the properties of fatty acids dictate the properties of fats/oils in which they are present

 To understand nutritional importance of EFA, MUFA and PUFA.

Concept Map

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3. Description

Fatty acids are constituents of all fats/oils and are also present in some other forms of lipids as we have been in the last module.

In organic chemistry, many books refer to carboxylic acids as fatty acids. This nomenclature came about because fatty acids which are part of fats/oils are carboxylic acids with a long chain. While chemistry of fatty acids is dictated by the functional group (-COOH group), we will learn in the next few modules that the nature of the chain also dictates much of the biology of fatty acids.

Fatty acids are part of many lipids. Notable among these are oils/ fats which are triglycerides.

While we will cover triglycerides in the next module, we already know their structure from the earlier module.

All the properties of fats/ oils: Physical, chemical, nutritional values etc are all decided by fatty acids. Fatty acid function is an important part of oleochemical industry.

Fatty acids have –COOH as the defining functional group with a side chain which mostly consists of hydrocarbon chain.

The most common fatty acids consist of a straight chain of hydrocarbons connected to the carboxyl group.

R-(CH₂)_n – COOH

The chain may be completely saturated or may contain one or more double bonds. The chain may have branches in some cases.

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Nomenclature

The simple nomenclature describes the chain length, the number and positions of double bonds (if any) and indicates the branch point (if any).

Again both IUPAC and common names are followed for naming fatty acids. In biochemistry, the use of common name is more prevalent.

Thus, biochemists tend to call

CH₃-(CH₂)₂ – COOH butyric acid. While in this case, butanoic acid is also used.

However, CH3-(CH2)14 – COOH is generally called just palmitic acid rather than calling it n- hexadecanoic acid. So, for higher chain fatty acids and more complex fatty acids, the use of common names is prevalent.

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Often the common names originated from the plant which produces a fat/oil/wax which is rich in that particular fatty acid. Like many scientific terms, the origin is a Greek or Latin word.

The C12 fatty acid is called lauric acid which is derived from Greek word „Laures‟ for laural plant.

The C14 fatty acid is Myristic acid as nutmeg in latin is called Myristica.

Similarly, we have Palmitic acid for Palm (latin Palmo) tree, stearic acid from the Greek word “Stear” which mean “hard fat”. Stearic acid is a C18 carboxylic acid with a melting point of 69.6 ^oC. It is a very common fatty acid in fats. The phrase “hard fat” refers to its high melting point. Its presence in a fat would make that fat solid even at moderately high temperature. It also tells us that ancient Greeks were familiar with the concept that fats melt at high temperature.

C₂₀ fatty acid Arachidic acid derives its name from the latin word „Arachis‟ which refers to the legume genus. Lignoceric (acid) has a composite of origin in latin words lignum (wood) + Cera (wax).

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The number of double bonds is indicated by writing the number of C in fatty acid, and stating the number of double bonds. The two numbers are separated by a colon. Hence Palmitic acid which contains 16 C and O double bond is described as 16:0.

Oleic acid is one of the commonest unsaturated fatty acid. Its name is derived from the Latin word oleum which means oil. This indicated that our knowledge of chemical structure of oils

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and fatty acid come much later. Oleic acid is an 18:1 fatty acid. Thus, oleic acid is stearic acid with one double bond in the chain.

Oleic acid with one unsaturated bond belongs to the class of fatty acids which are called MUFA (Mono Unsaturated fatty acids).

It is also a norm to indicate the position of double bond (s) with a superscript on  (delta).

This makes oleic acid as 18:1 (⁹) as C-19 is the smaller number of C-atom participating in double bond formation.

Another important MUFA is 16:1 (⁷) Palmitoleic acid. Palmitoleic acid is the MUFA corresponding to palmitic acid.

It is worth noting that both MUFA have double bond between C-9 and C-10.

In numbering omega fatty acids, the last C-atom is counted as ὠ-1. Thus ὠ-3 fatty acids have a double bond between ὠ-3 and ὠ-4 C-atoms.

We have been writing structures of all carboxylic acids in unionized form for the sake of simplicity. At physiological ~7, all fatty acid would occur as carboxylate anion as –COOH group ionizes at acidic pH. The exact pKa in each case of course, depends upon the structure of the individual fatty acid.

 The configuration around the double bond in most UFA is Cis and not trans.

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The trans configuration would be:

 Even in fatty acids having more than one double bond, the position of one of the double bonds is often between C-9 and C-10. However, it is not always so.

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 Some common fatty acids containing more than one double bond are Linoleic acid [18:2,

^9,12], α-Linoleinic acid [18:3, ^9,12,15] and Arachidonic acid [20:4, ^5,8,11,14].

 Please note that Arachidonic acid does not have a double bond between C-9 and C-10.

While in chemical nomenclature, the prefix poly is used for many, with fatty acids, all fatty acids with more than one double bond are clubbed together under the category of PUFA (Polyunsaturated fatty acid).

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 The double bonds in PUFA are never conjugated (alternate single and double bond) but invariably the carbon atoms participating in double bonds are separated by at least one –CH2- (methylene) group.

 Even in PUFA, all double bonds are in Cis-configuration. More is the unsaturation, lower is the melting point of the fatty acid and the corresponding triglyceride of which the fatty acid is a constituent.

 Thus, an oil would contain triglycerides formed from MUFA/PUFA, a fat is more likely to have triglycerides formed from saturated fatty acids.

 The chain length also dictates the melting point of the fatty acids. The lauric acid has a melting point of 44.2 ^oC which is about 25 ^oC less than that of stearic acid. Even palmitic acid which has just 2-C atoms less, has a melting point of 63 ^oC as compared to 69.6 ^oC of stearic acid.

 The PUFA thus show the synergy of the two factors. Arachidonic acid has a melting point of - 49.5^oC and is this liquid at room temperature. How those two factors

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influence the melting point is perhaps best illustrated by melting point of oleic acid [18:1, ⁹] at 13.4 ^oC and melting point of palmitoleic acid [16:1, ⁹] at -0.5 ^oC!

 This interdependence of melting point, chain length and unsaturation is not only relevant in the context of melting point of corresponding triglycerides [fats and oils] but as we will see has physiological relevance in many contexts.

 There are fatty acids with even a triple bond.

Ximenynic acid

Mycomycin

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 In Mycomycin, please note the presence of conjugated double bond which as we indicated earlier, is not a common structural feature of PUFA.

 The melting points are decided by the way fatty acids interact and pack. The saturated fatty acids have only single bond between various carbons in the chain. These single bonds do not hinder rotation and confer higher flexibility to the molecule. These chains also occur in extended form with no steric hindrance.

 The result is crystalline forms in which close van der waal‟s contacts between neighboring atoms are present. These structures, hence create fatty acids with higher melting points.

 In UFA, presence of double bond(s) in cis form disturbs regularity in packing. Hence intermolecular interactions are weaker in UFAs and PUFAs. This results in lower melting points of those fatty acids.

 The solubility of fatty acids is also decided by chain length. Simple short chain carboxylic acid, acetic acid, is freely soluble in water. The longer hydrocarbon chain, devoid of any potential for interaction with water, results in poor solubility in water.

 Lauric acid (12:0) has only 0.063 mg/g solubility in water. The long hydrocarbon chain reduces the effective overall interaction of water due to polar carboxylate anion.

 The limited solubility of lower homologues of fatty acids (like lauric acid) is due to –COO^- anion. Palmitic acid has much less solubility of 0.0083 mg/g of water. Stearic acid

with 2 more –CH2- groups, has merely 0.0034 mg/g solubility in water.

 Fatty acids, as simplest example of lipids, by definition are soluble in most of the organic solvents. Fatty acids, however, can be dispersed in water. The soap, which are sodium salts of fatty acid form micelles at a concentration of/ and beyond critical micelle concentration (CMC).

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Biological Importance of Arachidonic acid

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Structures of some prostaglandins

Arachidonic acid in humans is a precursor of a class of compounds known as eicosanoids. Eicosanoids (Greek: Eikosi means twenty) are C20 compounds. Prostaglandins are most well known example of an Eicosanoid.

Prostaglandins were discovered by Swedish scientist Ulf von Euler in the 1930s from human semen and were so named as he believed that these are produced by the prostrate gland. In fact, the semen prostaglandins are derived from the seminal gland.

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Prostaglandins are now known to be present in almost all tissues of both male and female animals.

Prostaglandins are oxygenated eicosanoids. Von Euler showed that these are hydroxyfatty acids.

Later with the availability of GC and mass spectrometry, Sune Bergstrom and jon Sjorall, the fellow Swedish scientists established the structures of some prostaglandins. Sune K. Bergstrom, Bengt I.

Samuelsson and John R. Vane were awarded Nobel prize in 1982 for their discoveries related to

“prostaglandins and the related biologically active compounds”.

It was discovered by John Vane in 1971 that aspirin inhibited the synthesis of prostaglandins which led to their immense pharmacological importance. This not only triggered extensive research on prostosanoids in general.

Arachidonic acid is the major precursor of eicosanoid hormones

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The prostaglandins as a class are derived from PUFA by enzymatic action of reductases and isomerases.

The various classes are designated by PGA to PGI and a subscript which denotes the number of C=C outside the ring.

The prostaglandins with a subscript of 2, that is, having two double bonds outside the ring are derived from arachidonate. Arachidonate can be converted into any of the three classews of compounds.

If acted upon by lipoxygenase, it forms leukotrienes. These compounds were first detected in leukocytes and have 3 double bonds. Action of cyclooxygenase instead converts arachidonate into either prostaglandins or thromboxanes.

Structures of some thromboxanes

Thromboxanes contain 6-membered ether ring. These were so named as they were isolated first from blood platelets (Thrombocytes). All thromboxanes have –OH group at C-15. Their nomenclature is similar to prostaglandins.

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Fatty acid precursors and related prostaglandins and thromboxanes

In most mammals, arachidonate is the PUFA present in plenty. Hence, PGE2 and TX2 are the most common eicosanoids. Arachidonate is derived from the membranes through the action of phospholipases A2. Alternatively, a phospholipases C action yields DAG. DAG acted upon by a lipase gives arachidonate. Arachidonate formation is the rate limiting step in the production of prostaglandins and thromboxanes.

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Synthesis of PGE2 from arachidonate

It was John Vane who also found that inhibition of prostaglandins by aspirin is due to aspirin inactivating prostaglandin synthase. This enzyme has two components: cyclooxygenase and hydroperoxidase.

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Cyclooxygenase introduces those O-atoms into prostaglandin from molecular oxygen O2. This leads to the formation of PGG2 in a 2e^- reduction process. The dioxygenase is a heme enzyme bound to the smooth endoplasmic reticulum of the cells.

Aspirin inactivates cyclooxygenase by acetylation of a serine, probably at or near the active site

Aspirin is acetyl salicylate. It reacts with the active site serine of the cyclooxygenase. The acetylated cyclooxygenase is inactive and cannot carry out the conversion of arachidonate.

Eicosanoids, including prostaglandins are called local hormones. These influence the activities of the cells which synthesizes them as well as neighbouring cells.

While normal hormones (insulin, adrenaline) may have a more uniform action, the actions of these local hormones are varied and depend upon the cell type.

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The discovery of cAMP which was called secondary messenger had created great excitement. Hence, the finding that prostaglandins act via cAMPrenewed that excitement. PGE, at a concentration of 10^-8 M inhibits the lipolytic action of epinephrine, glucagon, corticotrophin and TSH in adipose tissues. It does so by inhibiting adenylate cyclase in fat cells.

In other cells, prostaglandins on the other hand stimulate adenylate cyclase. Other physiological effects of prostaglandins include promoting inflammation (that is why aspirin is able to reduce inflammation), the regulation of blood flow to some organs, control of ion transports in membranes and synaptic transmission.

PUFAs

PUFAs stands for polyunsaturated fatty acids. As early as 1920s, the importance of linoleic acid and α-linoleinic acid in good nutrition was known and these were termed as essential fatty acids (EFAs)

These EFAs are not synthesized by animals (including human) and hence must be derived from dietary sources. This has led to modifying fats/oils to restructure these chemically or enzymatically to incorporate EFAs as constituent FAs.

The nutritional benefits of EFAs presumably arise from the fact that once consumed these are converted to long chain ω-3 and ω-6 fatty acids. In fact, there are two PUFA families: linoleic acid based and linoleinic acid based as these EFAs themselves are ω-3 and ω-6 FA respectively.

EFAs require both chain elongation and desaturation to generate long chain ω-FA metabolically.

Arachilonic acid (20:4) is an important ω-6 FA and eicosapentanoic acid (EPA, 20:5) and docosahexanoic acid (DHA, 22:6) are the most important ω-3 FA.

Fish oils are rich in ω-3 FA as they feed marine organism rich in ω-3 FA. While lot of nutritional benefits have been claimed for DHA, it is definite that infants need it for development of good vision and brain.

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MUFAs (Mono unsaturated fatty acids)

Not just PUFAs, MUFAs with oleic acid as the important example have shown health benefits.

American Heart Association recommended diet incorporated 12% MUFA.

Oleic acid is not an EFA as it can be produced by animals from desaturation of stearic acid. On the whole PUFAs show greater benefits in lowering serum lipids and cholesterol as compared to MUFAs.

Current improved varieties of sunflower, safflower, canola and even peanuts have oils rich in MUFAs.

In western countries, speciality frying and cooking oils have been commercially available. These are sold with the claimed enriched content of MUFAs. Examples include Nu-Sun^®, Clear valley^® and Trisun. Olive oil is rich in MUFA with78% of 18:1 Fatty acid.

Trans-Fatty acids (TFA)

Unlike MUFAs and PUFAs, oils/ fats containing trans-fatty acids have been found to be damaging health. The level of trans fatty acid in any hydrogenated fat depends upon hydrogenation conditions.

Ruminant fats such as present in milk, butter and tallow also contains a small amount of fat containing TFAs as a result of their own metabolism.

TFAs are reported to raise LDL. Fats/Oil containing high amount of TFA are called Trans-fats.

Margarine and many frying oils have high TFA content. Many processed food item generally contains high TFA content. Regulatory agencies are moving towards making labelling of Trans- fat content in food items compulsory in many countries. Many processed food items are now sold with the label of “0% trans fatty acids”. There is a growing teaching that consumption of trans fatty acids are more harmful than even saturated fats.

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The long chain R of fatty acids, RCOOH, dictates many important activities. In school organic chemistry, we are normally taught that organic chemistry is the chemistry of functional groups.

True, except in biochemistry the „R‟ also becomes important. We have seen that good fats/bad fats classification in nutrition content depends upon „R‟.

Summary

 Structures of various fatty acids

 Effect of long chain structure on various properties of fats/oils.

 EFA, MUFA, PUFA, ὠ-fatty acids and trans fats.